Ship stabilizers

ABSTRACT

A tank stabilizer for marine use has a high-Q tuned U-tank system in which the water levels in the two side tanks are boosted, when the system is operating at resonance, by a booster arrangement. The booster arrangement is insufficiently powerful to operate the stabilizer as an active stabilizer but improves the Q factor of the tank system to enhance its stabilizing power. The booster arrangement is controlled by roll-sensing means.

United States Patent Bell, John, deceased, late of Beckenham,

inventors [56] References Cited Kent, England, by Barclays Bank LimitedFOREIGN PATENTS and Bella Bell, Executors, Beckenham, 451 923 93 Great Bi i 1 14/ 125 Kent England Y OTHER RapEReii Appl. No. 796,649 Filed Jam,9 9 German Printed Applrcation No. 1,262,812 dated 8-1966 Patented May25, 1971 classified in class 114- 125 (Halden) Asslgnee Mull'headLimited Primary ExaminerTrygve M. Blix Kent, England AttorneyHolman,Glascock, Downing and Seebold Priority Feb. 2, 1968 v Great Britain5374/68 v ABSTRACT: A tank stabilizer for marine use has a high-Q tunedU-tank system in which the water levels in the two side 22 .2 3 tanksare boosted, when the system is operating at resonance, wing by abooster arrangement. The booster arrangement is insuffi- U.S. Cl 114/125ciently powerful to operate the stabilizer as an active stabilizer Int.Cl B63b 43/06 but improves the Q factor of the tank system to enhanceits Field of Search 114/122, stabilizing power. The booster arrangementis controlled by 125 roll-sensing means.

I 1 l 4 l X l l I I I l l SHIP STABILIZERS BACKGROUND OF THE INVENTIONThis invention relates to ship stabilizers, more particularly to tankstabilizers in which the motion of the mass of liquid is employed toproduce a torque for stabilizing the oscillatory motion of the ship.

Tank stabilizers for ships have long been known. In the case of rollstabilization such a stabilizer comprises two tanks respectivelydisposed in the port and starboard sides of the ship, with the tanksbeing partly filled with a liquid and connected at their bottom portionsby a liquid channel. The tanks may be connected at their upper portionsby a gas channel so that the gas above the liquid in the tanks is ableto flow between the tanks during rolling motion of the ship. In somearrangements the upper portions of the tanks may communicate withatmosphere. In practice, the liquid used in the stabilizer is water andthe gas is air.

Tank stabilizers can be considered as falling generally into twoclasses, active stabilizers and passive stabilizers.

The passive stabilizers were the first to be developed. They have theadvantage that they require no external power to be applied for theiroperation as they rely entirely on the rolling movement of the ship toalternatively fill the tanks of the stabilizer so that they provide acountering torque to suppress the rolling motion.

One such passive stabilizer was developed by Frahm and is the tunedU-tube stabilizer. The two limbs of the U are provided by the tanks atthe sides of the ship and the intermediate portion of the U provides aninterconnecting passage between the lower ends of the tanks. The wholesystem is tuned to a resonant frequency f which is normally the resonantfrequency f of the ship most likely to be encountered in practice. Whenthe ship is rolling at the resonant frequency the liquid in the U-tubesurges back and forth between the tanks with a phase relationship suchthat the synchronous rolling of the ship at the frequency f issuppressed by the 90 phase lag of the movement of liquid between the twotanks of the stabilizer.'

Although the turned passive stabilizer has theoretically certainadvantages it does have a practical disadvantage in that it sometimes isdifficult to achieve a desirably high Q-factor together with thenecessary stabilizing power, without an unacceptably large loss of themetacentric height GM of the ship due to the free surface effect of theliquid in the stabilizer tanks.

The active stabilizer utilizes an external source of power to controlthe movement of liquid between the two tanks of the stabilizer. Theactive stabilizer has the advantage that it may be used to suppress avery much wider range of rolling frequencies than can be suppressed bythe tuned passive stabilizer. As the control of the liquid flow betweenthe tanks is exercised entirely by an external source of power, theactive tank system normally has a natural resonant frequency lyingbeyond the range of rolling frequencies likely to be experienced by theship in practice. This ensures that there is no likelihood of thenatural resonant frequency of the tank SUMMARY OF THE INVENTION Inaccordance with the broadest aspect of this invention a method ofstabilizing a vessel carrying a high-Q U-tube tank stabilizer tuned tothe resonant frequency of oscillation of the vessel, comprises applyinga force to the tank liquid at a frequency substantially equal to theresonant frequency and in such a manner as to stabilize rolling motionof the vessel. I

In accordance with a second aspect of this invention a tank stabilizerhas a tank system which, when filled with liquid, has a resonantfrequency lying within the range of rolling frequencies likely to beexperienced by a ship into which the stabilizer is to be fitted, thesystem comprising two tanks having an interconnecting liquid passagebetween their lower end portions and having the liquid flow between themat the resonant frequency influenced predominantly by the rollingmovement of the ship and also by a booster arrangement controlled toassist the resonant liquid flow by imparting to the liquid a force whichvaries with rolling and which is controlled by a control system actuatedby roll-sensing means which respond to the rolling movement of the ship.

The booster arrangement enables the stabilizing power of the system tobe increased without a corresponding decrease in the metacentric height.

The tank stabilizer of the invention cannot be classified as an activestabilizer, as the booster arrangement is incapable of providing asufficient force to produce the necessary flow of liquid between thetanks in order to stabilize rolling motion of the shipl Neitherhoweveris the stabilizer a passive system as it does consume power during itsoperation. In practice the booster arrangement requires only arelatively small amount of power to operate it as the operation of thestabilizer relies primarily on the resonance of the system. The boosterarrangement merely provides a small force to make the operation of thestabilizer more effective.

The force applied by the booster arrangement preferably variescontinuously with the roll. However, the booster arrangement may bearranged to provide a continuous constant force during rolling, with theforce being reversed in direction at opposite ends of the roll.Alternatively, the booster arrangement may be arranged to provide aforce for a short duration at the ends of the roll so that the advantageof the invention is obtained when the force is integrated over aprolonged period of time.

The invention may also be used to enable the volume of liquid used inthe stabilizer and thus its free surface area to be reduced. Aconsequential increase of the metacentric height GM is then achieved andthe ship's stability is enhanced. v

The resonant'frequency of the tank system may be tuned to nearly thedesired resonant frequency of the vessel f by varying the effective areaof the liquid channel interconnecting the tanks. Such variation isuseful to allow forthe change in the vessel's rolling frequency as aresult of loading or other variations of the vessel's condition.

Preferably the booster arrangement utilizes an air pump connected in a.gas channel extending between the upper portions of the tanks. A pump ofcomparatively low power is all that is required and this can dischargein controlled manner into respective tanks by way of a suitable valveforming'the control system and whose position is actuated by therollsensing means.

Preferably the control system responds primarily to the rollacceleration of the ship.

Although the invention relates primarily to high-Q and therefore lowloss tank systems, in practice'there may be difficulties in getting theQ-factor of the tank which determines the sharpness of the responsecurve of the tank when plotted against frequency, as large as is desiredfor optimum stabilization. The Q of the tank must not be made too highas'this would have an adverse effect on the stabilization as the levelof the oscillation of the tank liquid would take too long to build up.It is therefore preferred to design the stabilizer so that O has itsoptimum value when it is equal to Q The quantity O the quality factor ofthe ship and-characterizing the damping of the ship in the sea with thetank stabilizer inoperative, maybe defined as'Q =fi/2a, where-Bis thepeak-to-peak roll angle caused by waves impinging on the ship at theresonant frequency of the ship f and a is'the median angle of slope ofsuch waves. In a ship with bilge keels Q,,,,,, is typically around whileif there are no bilge keels Q may be as high as 10. The factor Q may beincreased to some extent by rounding off edges inside the tank and bystreamlining the flow channels interconnecting them.

By constructing the stabilizer as two two-tank systems arranged onewithin the other it is possible to obtain certain ad vantages. Forexample, the two systems may be tuned to different frequencies lying inthe range of rolling frequencies experienced by the vessel, so that theoverall frequency response of the stabilizer has a humped band-passcharacteristic. The booster arrangement enhances the stabilizing powerof the two two-tank systems at the different rolling frequencies in thepassband. In some circumstances the use of two two-tank systems mayenable a better utilization of the available space to be achieved sothat an improved Q-factor is obtained for the combination.

In the case where the force applied by the control system is transmittedto the gas space above the liquid in the tanks by means of a pumparranged in a gas flow channel interconnecting the upper end portions ofthe tanks, a single pump working into highand low-pressure reservoirsconnected to its output and input sides, respectively, may be employedto drive a number of independent stabilizer tanks. Alternatively eachpair of tanks of each stabilizer may have its own pump with or withoutsuch reservoirs. The tops of the tanks may be indepen-.

dently connected together or/and to the atmosphere via a leak or leaksfor the flow of gas therethrough to prevent the air pressure aboverespective tanks providing a constant bias to the stabilizer.

The invention will now be described in more detail, by way of examples,with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates diagrammaticallyone form of tank stabilizer; and

FIG. 2 represents diagrammatically a second form of tank stabilizer.

, Referring to FIG. 1 a tank stabilizer 1 is mounted within a ships hulldiagrammatically shown in athwartships section at 2 and provided withbilge keels 3.

The stabilizer is of tuned U-tube type and comprises a pair of sidetanks 4 and 5 which are interconnected at their lower portions by anumber of parallel channels 6. The upper end portions of the tanks 4, 5are joined by a duct 7 provided with a lead valve 8 in the form of anadjustable constriction for equalizing the air pressure in the two tanks4, 5 when the stabilizer is not in use. The duct 7 is provided with abranch pipe 9 terminating in a vent valve 10 formed by an orifice ofsmall size through which excess pressure in the stabilizer can exhaustto atmosphere. The vent 10 also ensures that the air pressure in thestabilizer is the same as the outside atmosphere during times that thestabilizer is not operating. The stabilizer is filled with liquid to alevel indicated by the chain line X-X.

The tops of the tanks 4 and 5 are respectively connected by piping 12,13 to a booster arrangement shown generally at 14.

The booster arrangement 14 includes a pair of reservoirs 15, 16. Thereservoir 15 is connected to high-pressure output side 17 of an air pump18 while the reservoir 16 is connected to low-pressure input side of theair pump 18. The reservoirs l5 and 16 are also connected to outlets 20,respectively, through which airflow is controlled by a valve member 21having a pair of interconnected piston heads 22 disposed opposite theoutlets 20. The piston heads 22 are arranged on opposite sides of adiaphragm 23 through which the valve member 21 passes and which spansacross a working chamber 24 for the member 21. The ends of the workingchamber 24 are closed by diaphragms 25 and it is provided with threelaterally opening ports, 26, 27 and 28. The port 27 is connected to oneend of the pipe 12 while the other pipe 13 is connected to both of theports 26 and 28.

connected to the passages 12 and 13, respectively, or vice versa. Theinstantaneous position of the member 21 also determines the size of theorifice through which the reservoirs 15, 16 are connected to the pipes12, 13 so that the pressure of air supplied to the stabilizer 1 iscontinuously controlled by the roll-sensing means.

The parallel passages or channels 6 are of adjustable cross section toallow the response frequency of the stabilizer to be altered. The upperwall of each of the channels 6 is formed by a horizontal plate which isvertically movable by screwjacks 31. At opposite ends of the plate 130are respective flaps 32 spring urged upwardly to provide a smooth entryto each end of the channel. By raising and lowering the plate 130 theresonant frequency of the stabilizer system may be adjusted to a valuebest suited to the operating conditions of the ship, as thecross-sectional area of the channel is altered.

The tank stabilizer described operates in the following manner.

When the ship is not rolling, the output of the roll-sensing means movesthe member 21 to the position shown in FIG. 1 at which the outlets 20are both closed by the pistons 22. The reservoirs l5 and 16 are held atthe output high pressure and the inlet low pressure, respectively, ofthe pump 18 which is a continuously running constant volume pump.Alternatively, the pump 18 may be arranged, by suitable electricalconnections shown by broken lines, to be controlled in accordance withthe pressures within the reservoirs l5 and 16 and it may then be ofconstant displacement type. The leak valve 8 ensures that the air spacesabove the liquid in both of the tanks are at the same pressure which,because of the vent 10, is atmospheric pressure.

If the vessel begins to roll at a resonant frequency f which correspondsto the tuning frequency flank of the stabilizer, the roll-sensing meansstarts to move the member 21 in the working chamber 24 in sympathy withthe roll. The member 21 is reciprocated back and forth in harmonicfashion by the rollsensing means 30 and the amplitude of its movement isdetermined by the acceleration of the roll. This is a minimum when theship is on an even keel and is a maximum when the roll amplitude is amaximum.

The movement of the member 21 controls the extend of opening of theoutlets 20 so that air under pressure is fed in a continuouslycontrolled manner to one or other of the tanks 4 and 5. Simultaneously,the other tank is connected to the reservoir 16 which is held at apressure beneath atmospheric pressure so that a differential pressure isestablished having a continuous relationship with the roll of thevessel.

The differential air pressure acts to assist the operation of thestabilizer by building up the water levels at resonance so increasingthe stabilizing power of the tank. The bulk of the energy required tooperate the stabilizer is obtained form the rolling movement of the shipand only a relatively small amount of additional energy is required tooperate the pump 18.

If it is desired to alter the response frequency of the stabilizer witha change in loading of the ship, the screwjacks 31 are operated in theappropriate direction to raise or lower the plate 130.

A second embodiment of the invention will now be described withreference to FIG. 2. Parts of FIG. 2 identical with corresponding partsof FIG. 1 have the same reference numerals and will not be againdescribed. It will be understood that FIG. 2, like FIG. 1 is whollydiagrammatic.

The tank stabilizer shown in FIG. 2 comprises a pair of tank systemsarranged one within the other. Both tank systems are of high-Q factor,tuned U-tube type and have similar connections leading to a boosterarrangement 14 to allow the pressure above the liquid in the individualtanks of both systems to be varied. To prevent overcomplicating thedrawing, the connections associated with the outer tank system only areshown but it is to be understood that the connections associated withthe inner tank system and partly shown at 54, are similar. The two tanksystems are tuned, respectively, to different frequencies in the rangeof roll frequencies likely to be encountered by the vessel.

The first tank system comprises a pair of tanks 50, 51 disposed atopposite sides of the centerline of the ship and interconnected at theirlower ends by a passage 53.

The second tank system is formed by a pair of tanks 55, 56 disposedoutwardly of the tanks 50, 51, respectively, and interconnected at theirlower end portions by a number of parallel passages 57.

The tanks 55 is connected at its upper end portion by a pipe 60 topoppet valves 61, 62. The second tank 56 is connected by a pipe 63 toanother pair of poppet valves 64, 65. The poppet valves 61, 64 havestems 66 which rest on a reciprocable shaft 67 positionally controlledby roll-sensing means 68 similar in function and operation to theroll-sensing means 30 of FIG. 1. The shaft 67 is provided with twocamming flanges 70, 71. The pair of poppet valves 62, 65 have theirstems 72 also resting on the shaft 67.

Individual opening of the poppet valves 61, 65 allows air to be drawnpast them through a conduit 172 which extends to a reservoir 73 and tothe low-pressure inlet of a pump 74.

Individual opening of the other two poppet valves 64 and 62 allowscompressed air to flow through them from a conduit 76 connected tohigh-pressure outlet side of the pump 74 and also to an air reservoir77. Electrical connections shown by dotted line 78 control the boosterarrangement 14 so that the air pressures of the reservoirs 73 and 77 aremaintained at desired values.

The two tanks 55, 56 are interconnected at their upper ends by a channel7 having a branch pipe 9 and containing a leak valve 8. The branch pipe9 terminates in a vent valve 10. The parts referenced 7, 8, 9 andperform the same functions as correspondingly referenced parts in FIG. 1and will not therefore be again described.

The stabilizer shown in FIG. 2 operates in the following manner.

When the ship is on an even keel and not rolling, the rollsensing means68 shifts the shaft 67 to the position shown in FIG. 2 at which the fourpoppet valves are closed. The liquid or water levels in the two tankstabilizer systems assume equilibrium positions and the air pressuresabove all of the tanks are at atmospheric pressure. The pump 74 is thencontrolled solely by the reservoirs 73 and 77 to maintain the airpressures within them within desired limits.

If the vessel commences to roll at the frequency lying in the I band towhich the tank stabilizer is tuned, the roll-sensing means 68 respondsby reciprocating the shaft 67 to and fro. When the shaft 67 reciprocatesto the left, the two poppet valves 64 and 61 are opened, and when theshaft reciprocates to the right, the two poppet valves 62 and 65 areopened. The degree of opening of the poppet valves for any position ofthe shaft 67 is determined by the camming surface of the shaft and thecamming flange with which the stems of the poppet valves come intocontact. The stems of the poppet valves are resiliently biased towardsthe shaft 67 as is customary with the poppet-type valve.

When the pair of poppet valves 64, 61 is opened, the other pair ofvalves 62, 65 remains closed. The open valves allow air pressure to flowthrough the pipe 63 to the tank 56 and air to exhaust from the tank 55by way of the pipe 60 and the conduit 172 to the inlet side of the pipe74. The differential pressure acting on the two tanks 56, 55 causes thewater level in the tank 56 to be depressed and the water level in thetank 55 to be increased. The roll-sensing means so controls the shaft 67that the differential pressure acting on the tanks 55 and 56 assists thenatural movement of the water or liquid flow between the tanks 55, 56 atthe tuning frequency of the tank system. The other tank system formed bythe two tanks 50, 51 is operated in the same way.

As the two tank systems are tuned to different resonant frequenciestheir band-pass characteristic acts to suppress the roll of the shipover a range of frequencies. The force exerted by the differential gaspressure on the tanks 55, 56 varies with the acceleration of the roll sothat the tank system is able to provide an adequate torque forsuppressing the roll.

The pump 74 may be a centrifugal pump or a fan-type pump. Although adisplacement pump could be used it would tend to be wasteful of powerand would require special safety devices to avoid pressures building upinside the tanks.

The Q-factor of the tanks maybe increased to some extent by rounding offedges inside the tanks and by stream lining the flow channels and by theuse of other techniques known in the art for tuning tank stabilizersystems.

Various modifications of the above-described arrangements are possible.

For example, the ship may be provided with several tank stabilizers ofthe type shown in FIG. 1 or the type shown in FIG. 2. Each of the tankstabilizers could be provided with its own booster arrangement formed bythe pump and associated valve system. If desired each tank stabilizercould be provided with its own booster arrangement and the two tanks ofthe stabilizer which are to have the differential air pressure exertedacross them could be connected to the same air pump directly without theuse of reservoirs. On the other hand, a number of tank stabilizers maybe used which receive supplies of compressed air and low pressure fromtwo reservoirs in which the desired air pressures are maintained by asingle pump.

The roll-sensing means may employ a continuous electrical control suchas would utilize gyros and electrical roll-sensing devices, a hydraulicor pneumatic control, or ON/OFF impulse or any other control.

It is claimed:

1. In a tuned tank stabilizer including two tanks, means defining aliquid passage interconnecting the lower regions of said tanks, liquidcontained in said tanks and said passage imparting to said stabilizer aresonant frequency lying within a range of rolling frequencies likely tobe experienced by a ship into which the stabilizer is fitted, theimprovements comprising a power-consuming booster arrangement, means forimparting a force exerted by operation of the booster arrangement to theliquid in the tanks, a control system operably related to said means foradjusting said force, and roll-sensing means operably related to saidcontrol system for operating said control system in dependence upon therolling movement of the ship at said resonant frequency, whereby thepower to operate the stabilizer at said resonant frequency is derivedpredominantly from the rolling motion of the ship and is assisted onlyto a minor extent by the booster arrangement which is therefore designedto provide a limited power output insufficient to operate the stabilizeras an untuned active stabilizer.

2. In the tuned tank stabilizer as set forth in claim 1, the provisionof a booster arrangement defined by an air pump having a low-pressureinlet and a high-pressure outlet, a control system defined by a valvehaving an operating member moved rectilinearly by the roll-sensingmeans, and means defining airflow passages leading from the controlsystem to both tanks whereby the connection of one tank to the pumpinlet is accompanied by simultaneous connection of the other tank to thepump outlet.

3. A tuned tank stabilizer having two tanks each having upper and lowerportions, a liquid passage interconnecting said lower portions of saidtanks, a gas channel interconnecting said upper portions of said tanks,leak valve means in said gas passage, a branch pipe extending from saidpassage, vent means in said branch passage and opening to atmosphere,liquid in said tanks and said passage and imparting to said stabilizer aresonant frequency lying in a range of rolling frequencies likely to beexperienced by a ship into which the stabilizer is fitted, a boosterarrangement, pipe means extending from said upper portions of said tanksto said booster arrangement, an air pump in said booster arrangement andhaving a lowpressure inlet and a high-pressure outlet, a control systemin said booster arrangement and having a valve member rectilinearlymovable from an intermediate position to two extreme positions,roll-sensing means responsive predominately to the roll acceleration andpositionally controlling said valve member, and conduits providingseparate gas flow paths between said control system and saidhigh-pressure inlet and low-pressure outlet of said pump, whereby saidvalve member is displaced by said roll-sening means during rolling ofthe ship form said intermediate position at which it disconnects saidpump from said tanks and the extent of such displacement regulates theforce applied to the tank liquid by the differential air pressureexerted in the upper portions of said two tanks.

4. The tuned tank stabilizer as set forth in claim 3, wherein saidbooster arrangement includes a low-pressure gas reservoir connected tosaid pump inlet, a high-pressure gas reservoir connected to said pumpoutlet, and means controlling operation of said pump in accordance withthe pressures in said reservoirs.

5. A tuned tank stabilizer having a first tank system arranged within asecond tank system, each of said tank systems comprising two tanks eachhaving upper and lower portions, a liquid passage interconnecting saidlower portions of said tanks, a gas channel interconnecting said upperportions of said tanks, leak valve means in said gas passage, a branchpipe extending from said passage, vent means in said branch passage andopening to atmosphere, liquid in said tanks and said passage andimparting to said stabilizer a resonant frequency different from theresonant frequency to the other system and lying in a range of rollingfrequencies likely to be experienced by a ship into which the stabilizeris fitted, a booster arrangement pipe means extending from said upperportions of said tanks to said booster arrangement, an air pump in saidbooster arrangement and having a low-pressure inlet and a high-pressureoutlet, a control system in said booster arrangement and having a valvemember rectilinearly movable from an intermediate position to twoextreme positions, roll-sensing means responsive predominantly to theroll acceleration and positionally controlling said valve member, andconduits providing separate gas flow paths between said control systemand said high-pressure inlet and low-pressure outlet of said pumpwhereby said'valve member is displaced by said roll-sensing means duringrolling of the ship from said intermediate position at which itdisconnects said pump from said tanks and the extent of suchdisplacement regulates the force applied to the tank liquid in the twosystems which, being tuned to different frequencies, exhibit a band-passcharacteristic offering a range of frequencies over which the stabilizeris effective.

6. The tuned tank stabilizer as set forth in claim 5, in which saidbooster arrangement includes a low-pressure gas reservoir connected tosaid pump inlet, a high-pressure gas reservoir connected to said pumpoutlet, and means controlling operation of said pump in accordance withthe pressures in said reservoirs.

1. In a tuned tank stabilizer including two tanks, means defining aliquid passage intErconnecting the lower regions of said tanks, liquidcontained in said tanks and said passage imparting to said stabilizer aresonant frequency lying within a range of rolling frequencies likely tobe experienced by a ship into which the stabilizer is fitted, theimprovements comprising a power-consuming booster arrangement, means forimparting a force exerted by operation of the booster arrangement to theliquid in the tanks, a control system operably related to said means foradjusting said force, and roll-sensing means operably related to saidcontrol system for operating said control system in dependence upon therolling movement of the ship at said resonant frequency, whereby thepower to operate the stabilizer at said resonant frequency is derivedpredominantly from the rolling motion of the ship and is assisted onlyto a minor extent by the booster arrangement which is therefore designedto provide a limited power output insufficient to operate the stabilizeras an untuned active stabilizer.
 2. In the tuned tank stabilizer as setforth in claim 1, the provision of a booster arrangement defined by anair pump having a low-pressure inlet and a high-pressure outlet, acontrol system defined by a valve having an operating member movedrectilinearly by the roll-sensing means, and means defining airflowpassages leading from the control system to both tanks whereby theconnection of one tank to the pump inlet is accompanied by simultaneousconnection of the other tank to the pump outlet.
 3. A tuned tankstabilizer having two tanks each having upper and lower portions, aliquid passage interconnecting said lower portions of said tanks, a gaschannel interconnecting said upper portions of said tanks, leak valvemeans in said gas passage, a branch pipe extending from said passage,vent means in said branch passage and opening to atmosphere, liquid insaid tanks and said passage and imparting to said stabilizer a resonantfrequency lying in a range of rolling frequencies likely to beexperienced by a ship into which the stabilizer is fitted, a boosterarrangement, pipe means extending from said upper portions of said tanksto said booster arrangement, an air pump in said booster arrangement andhaving a low-pressure inlet and a high-pressure outlet, a control systemin said booster arrangement and having a valve member rectilinearlymovable from an intermediate position to two extreme positions,roll-sensing means responsive predominately to the roll acceleration andpositionally controlling said valve member, and conduits providingseparate gas flow paths between said control system and saidhigh-pressure inlet and low-pressure outlet of said pump, whereby saidvalve member is displaced by said roll-sening means during rolling ofthe ship form said intermediate position at which it disconnects saidpump from said tanks and the extent of such displacement regulates theforce applied to the tank liquid by the differential air pressureexerted in the upper portions of said two tanks.
 4. The tuned tankstabilizer as set forth in claim 3, wherein said booster arrangementincludes a low-pressure gas reservoir connected to said pump inlet, ahigh-pressure gas reservoir connected to said pump outlet, and meanscontrolling operation of said pump in accordance with the pressures insaid reservoirs.
 5. A tuned tank stabilizer having a first tank systemarranged within a second tank system, each of said tank systemscomprising two tanks each having upper and lower portions, a liquidpassage interconnecting said lower portions of said tanks, a gas channelinterconnecting said upper portions of said tanks, leak valve means insaid gas passage, a branch pipe extending from said passage, vent meansin said branch passage and opening to atmosphere, liquid in said tanksand said passage and imparting to said stabilizer a resonant frequencydifferent from the resonant frequency to the other system and lying in arange of rolling frequencies likely to be experienced by a ship intowhich the Stabilizer is fitted, a booster arrangement pipe meansextending from said upper portions of said tanks to said boosterarrangement, an air pump in said booster arrangement and having alow-pressure inlet and a high-pressure outlet, a control system in saidbooster arrangement and having a valve member rectilinearly movable froman intermediate position to two extreme positions, roll-sensing meansresponsive predominantly to the roll acceleration and positionallycontrolling said valve member, and conduits providing separate gas flowpaths between said control system and said high-pressure inlet andlow-pressure outlet of said pump whereby said valve member is displacedby said roll-sensing means during rolling of the ship from saidintermediate position at which it disconnects said pump from said tanksand the extent of such displacement regulates the force applied to thetank liquid in the two systems which, being tuned to differentfrequencies, exhibit a band-pass characteristic offering a range offrequencies over which the stabilizer is effective.
 6. The tuned tankstabilizer as set forth in claim 5, in which said booster arrangementincludes a low-pressure gas reservoir connected to said pump inlet, ahigh-pressure gas reservoir connected to said pump outlet, and meanscontrolling operation of said pump in accordance with the pressures insaid reservoirs.